Excitotoxic degeneration is initiated at non-random sites in cultured rat cerebellar neurons.

Prolonged stimulation of cultured cerebellar neurons by kainic acid (KA) leads to death of neurons first evident from the swelling of soma and neurites. Stimulation is accompanied by increases in [Ca2+]i and [Na+]i as monitored using digital imaging microfluorimetry. "Blebs" tended to form on neurites with the highest increases in [Ca2+]i. Points of Ca2+ entry into neurites via glutamate-receptor-gated channels predicted where approximately 80% of blebs would form tens of minutes later. These sites were close to neurite intersections where there was a high likelihood of synaptic contacts and were enriched in mitochondria as revealed by rhodamine 123 staining. Ca2+, but not Na+ entry, produced a loss of mitochondrial potential. Prolonged KA, but not 50K, applications could fully dissipate the neuronal Na+ gradient. Recovery of resting [Na+]i was delayed by Ca2+ loading. We propose that blebs form at certain synaptic regions due to localized ionic fluxes and local Ca2+ overloading. Increased [Ca2+]i may hamper restoration of normal [Na+]i permitting local osmotic swelling as well as activation of Ca(2+)-dependent enzymes and other processes. Na+ may slow, block, or reverse Na/Ca exchange and enhance swelling. These conditions could not be reproduced by global changes in ion concentrations produced by Ca2+ or Na+ ionophores. The earliest stages of excitotoxicity thus appear to be manifestations of localized disruptions of ionic homeostasis mediated by Ca2+ overload and Na+ influx.